The invention relates to the field of amplifier circuits, and in particular to a two-stage amplifier that can amplify both AC and DC voltage signals.
Many digital integrated circuits have been fabricated in CMOS technology with 5-volt supply voltage. This has led to a large number of systems with a 5-volt supply on the printed circuit boards. However, as CMOS technology has developed the maximum allowable supply voltage of the digital IC's has dropped to 3.3-volts for 0.5 μm and 0.35 μm technologies. In addition, it is expected that this voltage will be further reduced as newer technologies are developed. Since IC manufacturers want to use modern CMOS processes for cost reduction, a supply voltage compatibility problem results.
The publication entitled “Embedded 5 V to 3.3 V Voltage Regulator for Supplying Digital IC's in 3.3 V CMOS Technology”, IEEE Journal of Solid-State Circuits, Volume 33, No. 7, July 1998, Page 956–962, discloses a fully integrated voltage supply in which a 5-volt supply voltage is reduced to 3.3 volts on the chip. The lower supply voltage of 3.3 volts is used to supply a CMOS digital circuit designed for lower current consumption. In an embodiment of the voltage supply, the gate connection of an n-channel output transistor is controlled by a comparator via a charge pump to create the required activation voltage. The comparator and the n-channel output transistor, which is connected as a source follower, form a two-stage amplifier with an inverting and a non-inverting input. The charge pump is a voltage doubler whose input variable is the output voltage of the comparator. Because the prevailing load on the amplifier output is not known for stability reasons, the fed-back voltage is tapped from a simulator directly from the amplifier output. A linear regulator is not used to control the voltage doubler and thus to control the amplifier, but the fed-back voltage is merely tested, in the manner of a two-point control, whether it is greater or smaller than a specified comparison value. Depending on the difference, the charge pump increases or decreases the gate voltage of the n-channel output transistor. This arrangement is not very well suited for general amplification purposes.
Therefore, there is a need for an improved two-stage amplifier that is suited for reduced supply voltages.